1. Field of the Invention
The present invention relates to an optical, recording medium, and in particular, to an optical information recording medium capable of recording information therein, and reproducing the information therefrom by causing an optical change in, for example, transmittance and reflectance of a recording material for use in the recording medium when the recording medium is irradiated with a light beam. Further, the present invention relates to an optical recording medium of a write once read many (WORM) type, such as a WORM type optical disc, in particular, a digital video disc-recordable (DVD-R) for use in a high-density optical disc system using a semiconductor laser beam with a shorter wavelength.
2. Discussion of Background
The DVD-R has been developed as a next generation disc with a large capacity. The possibilities of increasing the recording capacity of the DVD-R are in the development of the recording material that can minimize a recording pit to be formed, introduction of image compression technologies such as Moving Picture Experts Group-2 (MPEG 2), and improvement in the method of shortening the wavelength of the semiconductor laser used to read the recording pit.
An AlGaInP laser diode with a wavelength of 670 nm is only one red semiconductor laser that has been developed and commercialized for a bar code reader and a measuring instrument. Along with the development of the optical disc with high recording density, the red semiconductor laser has been introduced and used in practice in the optical recording industry. For a driving system for a digital video disc (DVD), two types of laser diodes with different wavelengths of 635 nm and 650 nm are standardized as the light sources For attaining a higher recording density, a laser beam with a shorter wavelength is preferred. For a driving system for a DVD-R, 635 nm is a preferable wavelength for the light source. A reproduction-only DVD-ROM drive equipped with a light source with a wavelength of 650 nm or less is now commercially available.
Under such circumstances, the most preferable is a DVD-R that can record information therein and reproduce the information therefrom at a wavelength of 635 nm or less, or 650 nm or less only for the reproducing operation. Accordingly, there is an increasing demand for a recording material for use in the DVD-R, with excellent light resistance and storage stability and capable of recording and reproducing information by an optical pickup system using a laser beam having a wavelength of 650 nm or less.
As the recording materials for use in the DVD-R, a polymethine dye and an azo chelate dye have been conventionally developed.
A polymethine dye is disclosed as the recording material in Japanese Laid-Open Patent Applications 10-83577, 10-119434, 10-149583, 10-188339, and 10-278426.
Of the polymethine dyes, a cyanine dye and a squarylium dye have particularly excellent optical properties, and can provide adequate signal characteristics. However, the light resistance of the above-mentioned dyes is too poor to be used alone in practice. To compensate for the poor light resistance of the polymethine dye, there has been developed a light stabilizer which can work when used in combination with the polymethine dye. For example, the combination of a polymethine dye and a light stabilizer is disclosed in Japanese Laid-Open Patent Applications 10-109475, 10-109476, 10-134413, 10-151861, and 10-166739.
However, no combination has been found that can satisfy both the light resistance and the signal characteristics.
An azo chelate dye is disclosed as the recording material in Japanese Laid-Open Patent Applications 8-295811, 8-295812, 9-332772, 9-39394, 10-6650, 10-58828, 10-157293, 10-157300, 10-188340, and 10-188341. The light resistance of the azo chelate dyes is also poor although adequate signal characteristics can be obtained.
A formazan chelate dye is known to have excellent light stability. The application of the formazan chelate dye to the optical recording material is disclosed in Japanese Laid-Open Patent Applications 8-295079, 9-193546, 10-152623, and 10-154350. However, any of the above-mentioned conventional formazan chelate dyes exhibit the absorption in a long wavelength range, and therefore they are not suitable for the recording materials for use in the DVD-R.
The use of the formazan chelate dye as the light stabilizer for the cyanine dye has been examined. In general, the light stabilizing mechanism results from excitation transfer from the main dye to the light stabilizer. An effective light stabilizer is therefore required to exhibit absorptive power at such an energy level that is close to and lower than the excited energy of the main dye. In light of the above-mentioned light stabilizing mechanism, the absorption band of the formazan chelate dye is located at too long a wavelength to provide an adequate light stabilizing effect.
Furthermore, it is proposed to use any other dye and a metal reflection layer, as disclosed in Japanese Laid-Open Patent Applications 8-169182, 8-209012, and 9-58130.
An object of the present invention is to provide an optical recording medium with excellent light resistance and storage stability, especially suitable for the DVD-R which can cope with a high-density optical disc system using a semiconductor laser having a shorter wavelength.
Another object of the present invention is to provide an optical recording medium comprising a recording material which is highly soluble in organic solvents.
The above-mentioned objects of present invention can be achieved by an optical recording medium comprising a substrate, and a recording layer formed thereon comprising a formazan chelate compound selected from the group consisting of:
a compound represented by the following formula (I): 
wherein Z represents a group that forms a polyheterocyclic ring with a carbon atom and a nitrogen atom that are bounded thereto, A is an alkyl group, an aralkyl group, an aryl group, or cyclohexyl group, B is an aryl group, M represents a bivalent metal atom; and n represents the coordination number of formazan ligands;
a compound represented by the following formula (II): 
wherein Z represents a group that forms a polyheterocyclic ring with a carbon atom and a nitrogen atom that are bounded thereto, Axe2x80x2 represents a five-membered ring or six-membered ring, or a group that constitutes the heterocyclic ring, Bxe2x80x2 is an aryl group or an alkyl group, M represents a bivalent metal atom, and n represents the coordination number of formazan ligands; and
a compound represented by the following formula (III): 
wherein Z1 and Z2 are each a group that forms a heterocyclic ring with a carbon atom and a nitrogen atom that are bounded thereto, A1 and A2 are each an alkyl group, an aralkyl group, an aryl group, or cyclohexyl group, B1 and B2 are each an aryl group, p is an integer of 0 or 1, and when p is 1, X is methylene group, or sulfone group, and M represents a bivalent metal atom.
The formazan chelate compound of formula (I), (II) or (III) for use in the present invention can also serve as a light stabilizer when used together with the conventional recording dyes, such as a polymethin dye, squarylium dye, and azo chelate dye which can provide excellent signal characteristics in the DVD-R.